So the glass is down with that first coat of epoxy but it's been a week since you finished and now you're prepping for first fill coat? You can use sandpaper but I'd suggest starting with 120 or 150 grit, you want to avoid taking epoxy off the tops of the 'glass threads. 80's fine for sanding fillets that are to be recoated but a little too coarse for first sanding of 'glass I think.

Better might be to find some maroon "ScotchBrite" type pads, the non-woven abrasive stuff used for metal finishing. It's less aggressive than sandpaper, less risk of taking too much off and follows minute contours better than sandpaper.

Even after a week, depepnding on ambient temperatures where you're working, that first coat may not yet be fully cured so a chemical bond is still possible to some extent.

Taking off the glossy surface finish you have now will give the next coat a nice texture to mechanically bond to. (My take is there's some chemical bonding going on as well with open molecular ends exposed by the roughening of sanding/ScotchBriting, but I'm no epoxy chemist.)

Just be sure to avoid breathing the resulting dust from whatever you use to roughen the surface. Uncured epoxy can be a serious health hazard due to its chemistry.

Then remove any remaining dust by first vaccuming then wiping with damp paper towels or a clean sponge. Some advocate plain water for this, others suggest denatured alcohol; the goal is to get all the dust off before you begin applying the next coat.